In response to the rising of production capacity for silicon solar cell, the plasma enhanced chemical vapor deposition (PECVD) process for forming anti-reflection films is becoming the key process that required to be improved. That is, the plasma source used in the PECVD process has to be extended linearly in a direction perpendicular to the direction of conveyor movement. With linear extension of the microwave plasma source, the requirements including large-area plasma processing and high production capacity can be met. Thus, it is in need of an improved linear-type microwave-excited plasma source, which will be described in the present disclosure hereinafter.
Please refer to FIG. 1, which is the cross sectional sketch of a conventional linear-type microwave-excited plasma source disclosed in German patent DE 19812558A1. In FIG. 1, the conventional microwave-excited plasma source 100 comprises a reaction chamber 110, a quartz tube 120, a coaxial waveguide 130 and an inner conductor 135 therein. The coaxial waveguide 130 is disposed inside the quartz tube 120 while the quartz tube 120 is disposed inside the reaction chamber 110.
Thereby, when two microwave transmitters located at both ends of the coaxial waveguide 130 are used for launching microwaves from the two ends of the coaxial waveguide 130, the microwaves will travel inside the coaxial waveguide 130 and leaks out of the coaxial waveguide 130 by passing through the quartz tube 12 for exciting plasma 60 accordingly. The plasma meanwhile also plays a role of being the outer conductor of the coaxial waveguide 130. Then, by applying the plasma 60 on the surfaces of the silicon wafers 140, a thin-film deposition process is enabled.
Please refer to FIG. 2 and FIG. 3, which are two schematic diagrams showing the linearly distributed plasma excited by two different microwave powers launching from both sides of the coaxial waveguide. It is noted that the vertical axis in the Cartesian coordinate system in both FIG. 1 and FIG. 2 is used for representing plasma density and the horizontal axis is used for representing the linear positions. As shown in FIG. 1 and FIG. 2, the line n1 represents the linear plasma density distribution excited by applying one microwave power launching from the left end to the right end of the coaxial waveguide 130 whereas it is decaying to the right; and relatively, the line n2 represents the plasma distribution excited by applying the other microwave power launching from the right end to the left end of the coaxial waveguide 130 whereas it is decaying to the left. Thus, the actual plasma density n in the reaction chamber 110 is equal to the superposition of the plasma density of n1 and the plasma density of n2.
For the purposes of enlarging the area of plasma processing and raising the production capacity, linear extension of the liner-type microwave plasma source 100 is necessary. However, the longer the length of the liner-type microwave plasma source is extended, the less uniformly the linear plasma density will distribute. The reason is as follows. For linear extension of the plasma source 100, each of the two microwave powers will radiate to exhaust completely 130 far before reaching the other end of the coaxial waveguide 130 no matter each of them is being applied to the coaxial waveguide 130 through the left end or the right end. Therefore, the actual plasma density n in the reaction chamber 110, equal to the superposition of the plasma density n1 and the plasma density n2, will become less uniform; that is, the plasma density will be higher at the two sides in the reaction chamber 110 corresponding to the two ends of the coaxial waveguide 130 and lower in the middle, as shown in FIG. 2. Nevertheless, if there is a method to control the two microwave powers radiating into plasma 130 by making each of the microwave powers radiate to exhaust almost completely just before reaching the other end of the coaxial waveguide 130 no matter each of them is being applied to the waveguide 130 through the left end or the right end, the actual plasma density n in the reaction chamber 110 will become uniform as shown in FIG. 3. Although the uniformity of plasma density can be improved by increasing the input microwave powers from transmitters to make each of the microwave powers radiate to exhaust almost completely just before reaching the other end of the waveguide 130, however, arc-discharging happening at the two ends of the reaction chamber corresponding to two ends of the coaxial waveguide 130 will be exacerbated and is going to affect the stability of plasma excitation. In addition, because high-power microwave transmitters are very expensive, using such transmitters might be commercially uncompetitive.
Another issue is about maintenance. The quartz tube 120 soaking in the plasma will cause thin films being deposited on the outer surface of the quartz tube such that the coupling between microwave and plasma will change. Accordingly, the linearly distributed plasma density in the reaction chamber 110 will also be less uniform and thus adversely affect the quality of thin-film deposition on the silicon substrate 140.
Although the aforesaid problem can be solved by replacing the quartz tube regularly, such maintenance of replacement can be very time-consuming and consequently the production capacity may be reduced accordingly.
Therefore, it is in need of a linear-type microwave-excited plasma source using rectangular waveguide with a biased slot as the plasma exciter for overcoming the aforesaid problems.